In the course of quality assurance in the printing industry and, at the same time, under the pressure for cost reduction, it is known to simulate the printing process on a printing machine on a different appliance before the actual edition print.
This other appliance is nowadays conventionally a colour printer which, by suitable calibration by means of colour profiles in drive software, is made capable of producing a print-out which is significantly more cost-effective than would be possible with the printing machine, but at the same time has essential features of the subsequent print on the printing machine.
The result of printing print data on an appliance which simulates specific characteristics of the print subsequently produced on the printing machine is referred to in the printing industry as a proof, the procedure itself is referred to as proofing.
Expressed in another way, a proof is understood to mean a test print as a replacement initial print, primarily for the simulation of four-colour prints.
In order to distinguish better from the result of printing on the actual printing machine, that is to say the print, the term proof will be used below for the test print, that is to say for the printing of the printing data on an appliance which simulates specific characteristics of the print subsequently produced on the printing machine.
For most methods in the printing industry, the printing data provided for the reproduction are screened before the production of the printing plate.
This screening is necessary in order to be able to reproduce tonal value graduations in the print. The reason for this is that the ink to be printed in the printing machine itself does not have any graduations.
For this reason, the amount of ink which is printed onto a specific area of the printing material is controlled via a raster.
The general operating sequence during printing will be represented below using a block diagram 500 shown in FIG. 5.
Unscreened printing data 501 are projected onto screened printing data 503 by means of a raster image processor 502 (RIP). The screened printing data 503 are printed by a printing machine 504, by which means a print 505 is produced.
The unit designated as a printing machine 504 may be a digital printing machine or a printing machine which is coupled to a film exposer and printing-plate copier or to a digital printing-plate exposer. For the unit, it is important only that the unit converts the screened printing data 503 into the final result, the print 505.
The methods for proofing printing data according to the prior art can be divided up into three groups:    a.) methods which operate with unscreened printing data;    b.) methods which operate with screened printing data;    c.) methods which operate with screened printing data and remove the raster from the printing data again by means of a descreening method, as it is known.
The following advantages and disadvantages result from the respective procedures, which will be explained in more detail below:
a.) Methods Which Operate with Unscreened Printing Data (cf. FIG. 6a and FIG. 6b):
In this method, as shown in the block diagram 600 in FIG. 6a, the unscreened printing data 501 are fed to a proofer 601 and, in a colour management system, as it is known, are subjected by the latter to a colour space transformation and then screened in a suitable way for the proofer 601 and output by the latter, by which means the proof 602 is produced.
With the proofing method using unscreened printing data 501 it is possible with good quality to simulate the colour response during screening and printing on the printing machine 504.
Given appropriate calibration by means of colour profiles, the results, that is to say in the proof 602, agree so precisely with the print 505 in terms of coloration that the proof 602 is referred to as true to colour. This means that the proof 602 can be used for assessing the colour quality of the subsequent print 505.
FIG. 6b shows an example 603 of a proof 602 for unscreened printing data 501, according to this example 603 a 50 percent tonal value before screening.
The drawback with this procedure is, in particular, that the printing data likewise have to be screened in order to be output on the proofer 601. However, this screening on the proofer 601 does not correspond to the screening on the printing machine 504 and, at best, may be similar to the latter.
This means that a proof 602 created with the aid of the unscreened printing data 501 is not able to provide the user with any information about the quality of his screening.
Faults which arise during the screening cannot be detected with such a proof 602.
Possible faults are, for example, undesired patterns in the individual colours or a Moiré, as it is known, when the colours are overprinted.
b.) Methods Which Operate with Screened Printing Data (cf. FIG. 7a and FIG. 7b):
In a proofing method which uses screened printing data 503, the unscreened printing data 501 are projected onto screened printing data 503 by means of the raster image processor 502 (RIP). The screened printing data 503 are fed to a proofer 701 and output by the latter, by which means the proof 702 is produced. According to this procedure, separate screening in the proofer 701 is not necessary (see block diagram 700).
One advantage of this procedure is to be seen in particular in the fact that the screening, as will subsequently also be visible on the print 505, is shown to the user on the proof 702.
However, one disadvantage of this procedure is to be seen in the fact that it is not possible to achieve a true-to-colour-proof 702.
The reason for this is, in particular, that during the screening operation, all the tonal values (luminance values and/or chrominance values) are reduced to only two tonal values, namely a first tonal value with “0” percent application of colour and a second tonal value with “100” percent application of colour.
Intermediate steps no longer exist (cf. FIG. 7b, in which a 50 percent tonal value 703 is shown after screening).
During a colour space transformation to be carried out, only these two colour values would be transformed. As a result, however, no matching to the colour response of the proofer can be achieved any more, since intermediate tones would necessarily also be required for this purpose.
c.) Methods Which Operate with Screened Printing Data and Remove the Screen from the Printing Data Again by Means of Descreening Methods, as They Are Known (cf. Block Diagram 800 in FIG. 8):
In such a method, the screened printing data 503 are subjected to descreening 801, and the unscreened printing data 802 thus formed are fed to a proofer 803 and output by the latter, by which means the proof 804 is produced.
Expressed in another way, the raster information is calculated from the image again (the screened printing data 503), in order to recover the original, unscreened information (unscreened printing data 501, 802). Using the unscreened printing data 802, the procedure is then that described under point a).
The procedure according to point c) therefore has the same advantages and disadvantages as the procedure according to point a).
Such a method is generally used when there is no possibility of tapping off the unscreened printing data 501 in an existing operating sequence, but the main focus is the colour fidelity.
The invention is based on the problem of specifying a method and an apparatus for proofing screened printing data while maintaining the raster information, with which the colour fidelity of the proof, as compared with the print to be produced, is improved with respect to the known method.
The problem is solved by the method and the apparatus for proofing screened printing data while maintaining the raster information having the features according to the independent patent claims.